Sweetening petroleum hydrocarbons and method for regenerating the treating solution



y 1 w J. PIETERS ET AL 2,393,951

SWEETENING PETRC ELEUM HYDROCARBONS AND METHOD FOR REGENERATING THETREATING SOLUTION Filed May 28. 1956 9O 8O 7O 6O 5O 4O 3O 20 IO CRESOLINVENTORS WILLEM J. PIETERS 2 CORNELIS o. TEN HAVE THEIR ATTORNEYSWEETENING PETROLEUM HYDROCARBGNS AND METHOD 1? OR REGENERATHNG THETREATING SQLUTION Willem ll. Pieters and Cornelis D. Ten Have,Amsterdam, Netherlands, assignors to Shell Development Company, NewYork, N.Y., a corporation of Delaware Application May 28, 1556, SerialNo. 587,633

Claims priority, application Netherlands May 31, 1955 6 (Ilaims. (*Cl.208-204) This invention relates to a method for the direct sweetening ofhydrocarbon oils containing acidic sulfur compounds. More particularly,it relates to a method for converting acidic and malodorous mercaptansin petroleum hydrocarbon fractions into organic disulfides, andespecially to such a method for the treatment of petroleum hydrocarbonfractions which contain a relatively small proportion of suchundesirable sulfur compounds.

It is well known commercial practice to extract mercaptans fromhydrocarbon oils by means of an aqueous solution of an alkali metalhydroxide and a solutizer such as an alkali metal phenolate and/ or analkali metal organic carboxylate, such as naphthenates, butyrates, andthe like. The resulting spent caustic extraction solution is usuallyregenerated by stripping the mercaptans therefrom, as by steamstripping, or by oxidation with air, usually in the presence of anoxidation catalyst.

It is further known (cf. US. Patents 2,015,038 and 2,550,905) that themercaptans present in hydrocarbon oils can be oxidized to disulfides ifthe hydrocarbon oil is brought into contact, in the presence of anoxidizing agent, for example, oxygen, with an alkali metal hydroxidesolution containing a comparatively small amount of a special type ofphenol which is active as oxidation catalyst, particularly a polyhydricphenol or aminophenol having the hydroxyl groups or the hydroxyl andamino groups in the orthoor para-position with respect to each other.According to US. Patent 2,550,905, the aqueous alkali metal hydroxidesolution may also contain cresols and xylenols which promote thesolubility of mercaptans in the aqueous alkali metal hydroxide solution.

With methods by which mercaptans present in hydrocarbon oils,particularly in gasoline or kerosene, are oxidized to disulfides bycontacting the oil with an aqueous alkali metal hydroxide solution inthe presence of oxygen, the oxidation itself takes place in the aqueousalkali metal hydroxide solution. The mercaptans are first extracted fromthe hydrocarbon oil by the alkali metal hydroxide solution and in thissolution, in which they are present as mercaptides, they are oxidized bythe oxygen present to disulfides which then pass into the hydrocarbonoil. At the same time, the oxygen, being much more soluble in the-oilthan in the aqueous solution, is supplied to the aqueous solution forthe oxidation primarily via the oil. However, the transfer of oxygenfrom the hydrocarbon oil to the aqueous alkali metal hydroxide solutionproceeds with relative difiiculty.

A method is described in a copending application, now I Ten Have, U.S.2,763,594, by which the oxidation of mercaptans to disulfides inprocesses of the type referred to in the preceding paragraph is elfectedmuch more rapidly. This method involves the use of aqueous alkali metalhydroxide solutions containing phenolates and having a water content notgreater than 54% by volume. This method has been widely adopted incommercial practice because of its effectiveness and simplicity,especially in the sweetening of hydrocarbon fractions containingPatented July 7, 1959 an amount of mercaptans which is not so great asto justify the relatively expensive mercaptan extraction processeswherein a large volume of extraction solution must be applied to thehydrocarbon oil and subsequently treated or regenerated in a separatestep for the removal of the extracted mercaptans.

In this previous process, the treating solution is gradually dilutedwith water of reaction formed during the oxidation of rnercaptans todisulfides as well as by Water which is usually present in smallquantities in the hydrocarbon oil being treated. Thus, while thesolution need not be subjected to the major stripping or regenerationsteps necessary in mercaptan extraction processes for the removal oflarge quantities of mercaptans, nevertheless, the water of dilution mustbe removed continuously or intermittently to avoid dilution of thesolution beyond the limit of 54 percent by volume whereupon themercaptan oxidation reaction is drastically slowed. Heretofore, it hasbeen necessary to vaporize the water with which the solution has beendiluted, an expedient which is relatively expensive because it involvesthe heating of the entire treating solution to the vaporizingtemperature and the use of stripping steam or the like. Also, in thatprocess the oxygen which is generally present during the regeneration,and the high temperatures involved, operate to convert some of the alkylphenolate present to highly acidic oxidation products which irreversiblycombine with some of the alkali metal hydroxide present, thus causing aloss of both alkali metal hydroxide and phenolate.

It is therefore a principal object of the present invention to providean improved process for the sweetening of hydrocarbon oils. Anotherimportant object is to provide an improved process for the conversion ofmercaptans to disulfides in an aqueous caustic solution. A furtherobject is to provide an improved process for the conversion ofmercaptans to disulfides by the use of an aqueous alkali metal hydroxidesolution which can be easily and inexpensively regenerated. Otherobjects will be apparent in the description of the invention.

It has now been discovered that the foregoing objects are accomplishedby contacting a light hydrocarbon oil containing mercaptans, in thepresence of oxygen, with an aqueous treating solution of an alkali metalhydroxide and a phenolate, the initial composition of which is the sameas that of an aqueous solution of an alkali metal hydroxide and aphenolate which can co-exist as a separate liquid phase in heterogeneousequilibrium with one or more solid compounds of the alkali metal of thealkali metal hydroxide with the phenoxy radical of the phenolate.

Further description of the invention will be made with reference in partto the accompanying drawing, wherein:-

Figure I is the phase diagram of the ternary system water-potassiumhydroxide-cresol at 20 C., and is exemplary of the water-alkali metalhydroxide-phenol systerns suitably used in the practice of theinvention. The numerals along the axes of the diagram are thepercentages by weight of the various components.

Figure II is a flow diagram of a typical application of the process ofthe invention.

The definition of the treating solutions suitable in the practice of theinvention means in effect that the process is carried out with solutionswhich constitute only an extremely small part of the ternary phasediagram for a water-alkali metal hydroxide-phenol system. For example,in Figure I, the area ABCDE represents solutions which can be used inthe practice of the process of the copending application, now Ten Have,US. 2,763,594, referred to hereinbefore. However, in the practice of thepresent invention, for this particular system, only solutions are usedin contacting the hydrocarbon oil which lie on the lines ED and DCbecause onlysuch solutions can co-exist as a separate liquid phase inheterogeneous equilibrium with a solid compound of the alkali metal andthe phenol.

The process of the invention, whereby a mercaptancontaining lighthydrocarbon oil is contacted in the presence of oxygen with a treatingsolution as defined above, is advantageous for several reasons. In thefirst place, the oxidation reaction by which mercaptans are converted todisulfides takes place in the aqueous treating solution. The speed ofthe reaction is thus determined by the rate the mercaptans aretransferred into the treating solution and also by the rate the solutioncan absorb the oxygen required for the reaction. The treating solutionsused in the process of the invention are superior to more dilutesolutions used heretofore because the distribution coefiicient, i.e.,the ratio of the equilibrium concentration of the mercaptan sulfur inthe aqueous phase to that in the hydrocarbon oil phase, is much higherfor the present solutions. Also, the rate of transfer of oxygen intosuch solutions is much higher. As a result, a smaller quantity of thetreating solution is required for treating the same amount ofhydrocarbon oil than was heretofore the case, for example, half or evenless, so that with existing contacting equipment more hydrocarbon oilcan be sweetened.

Still another advantage of the invention is that the regenerating agentwhich may be used to remove excess water from the treating solution willnot absorb hydrocarbons, so that provision for hydrocarbon recovery inany evaporation step is not necessary as it would be if the water ofreaction were removed by direct evaporation of a treating solutionaccording to prior processes.

More especially, however, the regeneration of the treating solutions ofthe invention is considerably simplitied and less expensive. Instead ofvaporizing excess water from the used treating solution, with theattendant high cost of supplying heat and the loss of alkali metalhydroxide and phenolate because of the oxidation of phenolates at thehigh regeneration temperatures, as was heretofore necessary, the usedtreating solution of the process of the invention is regenerated merelyby contacting it with a solid compound derived from the alkali metalhydroxide and the phenol, which can co-exist as a separate phase inheterogeneous equilibrium with the original treating solution. Thissolid compound is aptly referred to as the regenerating agent because bycontacting the used treating solution with such a compound, the treatingsolution reverts to its original composition, the excess water beingremoved therefrom by absorption into the regenerating agent to form, inpart, another solid compound containing the absorbed water as water ofcrystallization.

. This will be better understood by referring to Fi ure I. A preferredtreating solution in accordance with the invention has a composition onthe lines ED and DC, for example, at point G. This solution will bediluted with excess water during the treatment of themercaptancontaining light hydrocarbon oil, as discussed above. Thecomposition of the treating solution will thus shift from point G to apoint in the direction of the water apex of the phase diagram.However,when the used treating solution containing the excess water iscontacted with solid potassium cresolate, which has a compositionindicated by the point P, the excess water is removed from the usedtreating solution and its composition again reverts to that of theoriginal solution at point G. During this regeneration step, thecomposition of the regenerating agent will, of course, change in thedirection of the water apex of the phase diagram. After the solidcompound has absorbed sufiicient water of crystallization from the usedtreating solution to render it ineffective for further use in theregenerating step, it is replaced by a further amount of solid compoundcontaining little or no water of crystallization. The exhausted.regenerated agent is then reactivated by removing the absorbed water ofcrystallization therefrom by any of the conventional techniques, forexample, by heating to vaporize the water.

It will be noted that the reason that the composition of potassiumcresolate (F) lies outside of the phase diagram of Figure I is that itis calculated by subtracting one mol of water from a mol per mol mixtureof potassium hydroxide and cresol, and thus giving a negative watercontent in respect to the system waterpotassium hydroxide-cresol.

The alkali metal hydroxide used in the process of the invention ispreferably either sodium hydroxide or potassium hydroxide. Potassiumhydroxide is particularly preferred because of the lower viscosity ofsolutions thereof at the concentrations required in the process.

The alkali metal phenolates (phenates) in the alkali metal hydroxidetreating solution can be derived from unsubstituted phenol or frommonohydric alkylphenols, such as phenol, o-, mand p-cresols, the variousxylenol isomers, the ethylphenols, the propylphenols, and mixtures ofany of these, the alkyl groups of which contain no more than threecarbon atoms in total; these phenols are understood to be otherwiseunsubstituted. Thus, the suitable phenolates are those monohydroxyphenolates which contain only alkali metal, carbon, hydrogen and oxygenatoms and which contain from 6 to 9 carbon atoms. Such individualphenolates and alkylphenolatcs and mixtures thereof are referred tothroughout the description of the invention by the generic designationphenolate. Of the suitable phenolates, the cresolates and xylenolatesare preferred, especially the cresolates. It is also preferred that thealkali metal of the phenolate be the same as the alkali metal of thealkali metal hydroxide.

The solid compound with which the treating solution is initially capableof existing in heterogeneous equilibrium is a compound of (A) the alkalimetal of the alkali metal hydroxide of the treating solution, with (B)the phenoxy radical of the phenolate. In this connection the termphenoxy as used herein is meant to be analogous in scope to thephenolates defined above. Thus, it includes not only the phenoxyradical, C H O--, but also the oxy radicals of the other suitablephenolates, i.e., it also includes the cresoxy, xyloxy and C -alky1-phenoxy radicals, CH C H O-, (CH C H O and (CH C H O--, respectively.

This compound, as the regenerating agent, preferably contains no Waterof crystallization as it is first used in regenerating the treatingsolution, but some water of crystallization may be present initially aslong as the compound contains less than the saturation amount of waterof crystallization and is therefore able to absorb additional amounts ofwater from the used treating solution. At the time it is replaced withfresh regenerating agent, the compound may of course be fully saturatedwith water of crystallization. It will then still be in equilibrium withregenerated treating solution, but will be incapable of regeneratingadditional amounts of used treating solution.

The ratio of the volume of the alkali phase to the volume of thehydrocarbon oil in the contacting step may vary within wide limits andusually lies between 0.03 and 3; a ratio of 0.03 to 0.3 is preferablyused, particularly 0.05 to 0.1.

The oxidation step of the present process, i.e., the contacting of themercaptan-containing hydrocarbon with the treating solution in thepresence of air, is generally carried out at temperatures of 0 C. to 70C., prefer ably 10 C. to 45 C.

The oxygen for the oxidation step may be supplied to the two-phasesystem to be treated either as such or in the form of a mixture ofoxygen with another gas which is inert under the reaction conditions,e.g., in the form of air. Oxygen-yielding compounds such as ozone orperoxides may also be introduced into the two-phase system to betreated. Some of these possibilities may, of course, be combined.

TW A

greener If oxygen is used as such, it may either be dissolved in thehydrocarbon oil in advance, or be injected into the oil while the latteris being brought into contact with the aqueous solution of alkali metalhydroxide and phenolate. The oxygen is preferably present in an excessof 50% to 200%, particularly 75% to 125%, over the quantity of oxygentheoretically required for the conversion of the mercaptans in thehydrocarbon oil to disulfides. If peroxide is also used, for instance,in a quantity of to 40% of the stoichiometric quantity with respect tothe mercaptans or mercaptides to be converted, in accordance with theprocess of Pieters, US. 2,744,054, the quantity of oxygen supplied assuch can be considerably reduced. In this connection it should be notedthat hydrogen peroxide is not readily soluble in hydrocarbon oils andrapidly decomposes in the aqueous alkali metal hydroxide solution. It istherefore preferably injected into the hydrocarbon oil in the form of aconcentrated solution in water or mixed with the oil in the form of analcoholic solution. Organic peroxides may frequently be directlydissolved in the hydrocarbon oil, while ozone mixed with oxygen or airmay be supplied, for instance by first passing the oxygen or air throughan ozonizer.

If the process is used for removing mercaptans from gasoline or kerosenewith a mercaptan sulfur content of not more than 0.04% to 0.5% by weightand if the gasoline or kerosene is in equilibrium with atmospheric air,the quantity of oxygen present in the gasoline or kerosene willgenerally be suflicient to effect the desired oxidation. However, themercaptans are usually removed from the hydrocarbon oils shortly afterthe latter have been obtained from the crude oil and after any otherpre-treatments have been carried out, with the result that they are notsaturated with air. In this case it is often necessary for air oranother oxygen-containing gas to be dissolved in the hydrocarbon.

In general, the process is carried out at atmospheric pressure. If theprocess is used for removing mercaptans from hydrocarbon oils with arelatively high mercaptan sulfur content, e.g., more than 0.05% byweight, and air is used as the oxygen-containing gas, it may beadvisable to operate at elevated pressure in order to dissolve asutficient quantity of oxygen in the hydrocarbon oil. If peroxides arealso used, the increase in pressure may be considerably less than whenthese compounds are not used.

In order to promote the transfer of the oxygen from the hydrocarbon oilto the aqueous alkali metal hydroxide solution containing the phenolate,care must be taken to effect an intense contact between the two phases.This contact may be brought about, for example, by means of a propellermixer, a centrifugal mixer such as the so-called tnrbomixer (see John H.Perry, Chemical Engineers Handbook, 1941, pp. 1554-1555), or a colloidmill, for instance the so-called Hurrell mill. The alkali metalhydroxide solution may also be sprayed very finely into the hydrocarbonoil under high pressure, for instance, by means of a spray nozzle, orthe hydrocarbon oil may be sprayed in this way into the aqueous alkalimetal hydroxide solution.

The various agents customarily employed in extractions for increasingthe surface between the phase to be extracted and the extraction agentmay also be used for the present purpose. Thus, the process may becarried out in a column provided with packing, projections or rotatingdiscs. In this connection, it should, however, be borne in mind thatsudden drops in temperature of the alkali metal hydroxide solutionshould be avoided, since in this case there is a risk of a second liquidalkali phase forming, or of solid components crystallizing from thesolution.

The process can be applied for removing mercaptans from lighthydrocarbon oils, i.e., hydrocarbon oils with a boiling point or finalboiling point of at most 370 C., particularly gasoline and kerosene, ofdifferent origin,

including gasoline and kerosene obtained by straightrun distillationfrom crude oils, as well as gasoline and kerosene obtained from heavybasic materials by cracking. The so-called reformed gasoline may also befreed from mercaptans according to the present process. When using theprocess for hydrocarbon oils containing unsaturated components,especially cracked gasoline and reformed gasoline, it is preferred toadd to the oil an anti-oxidant such as an aryl amine or an alkyl phenol,the alkyl groups of which contain a total of 4 or more carbon atoms, toprevent the formation of peroxides and gum from the unsaturatedcomponents of the oil. In general, a quantity of 0.0001% to 0.01% byweight of such an anti-oxidant is sulficient.

It is often desirable to remove from the hydrocarbon oils any acidspresent therein, such as hydrogen sulfide, which are stronger than themercaptans by means of a dilute aqueous alkali metal hydroxide solution,before oxidizing the mercaptans according to the present process.Particularly with products obtained by catalytic cracking apre-treatment with dilute caustic alkali solution has the furtheradvantage that aromatic mercaptans, which are more diflicult to oxidizethan aliphatic mercaptans, are removed at least to a considerableextent. It is preferable to carry out this pre-treatment before thecracked products come into contact with oxygen so as to prevent gulmformation.

Since when applying the process for removing mercaptans from hydrocarbonoils the disulfides formed during oxidation pass again into thehydrocarbon oil, the process is primarily suitable for treating lighthydrocarbon oils with a low mercaptan content, viz. lower than 0.05 byweight, and preferably lower than 0.02% by weight, calculated asmercaptan sulfur.

When gasoline or kerosene with a considerable mercaptan sulfur content,e.g., 0.05 by weight or more, is to be freed from mercaptans, thegreater portion of the mercaptans, if desired together with other sulfurcompounds, may be first removed by any of the hitherto usual methods,and then the remainder of the mercaptans oxidized according to theprocess of the invention. A more advantageous application of theinvention to such high mercaptan-content oils is to use a two-stepintegrated process, the first step being a conventional mercaptanextraction with a relatively dilute extracting solution of an alkalimetal hydroxide and an alkali metal phenolate whereby most of themercaptans are removed and either stripped from or oxidized in thedilute solution, and the second step being the use of the concentratedsolutions defined herein for the oxidation of the remaining mercaptans.In this embodiment of the invention the mercaptan-containing dilutesolution from the first step is stripped or oxidized for the removal orconversion of the mercaptans; water is removed by then contacting thesolution (and equilibrating it) with a solid compound as defined herein;the thus-reconcentrated solution is used in the second step oxidation ofthe remaining mercaptans; and finally the solution from the second step,which now contains excess water, is recycled to the first step and thereused as the relatively dilute extracting solution.

The present process provides a very simple method by which lighthydrocarbon oils can be freed from mercaptans in a short time, which inmost cases varies between 2 and 20 minutes. If the hydrocarbon oilcontains mercaptans which are diificult to oxide, it may be necessary tokeep the oil and the aqueous solution of the alkali metal hydroxide andphenol-ate in contact with each other in the manner described for asomewhat longer period. With a sufficiently intense contact between thehydrocarbon oil to be treated and the aqueous solution of alkali metalhydroxide and phenolate, it is, however, also possible in the lattercase to free the hydrocarbon oil from mercaptans to such an extent thatthe oil has a negative doctor tes within an hour.

The invention is further illustrated by the following ex- 70 cu. m. ofgasoline with a mercaptan sulfur content of 0.02% by weight, 4 cu. m. ofcresolate-containing potassium hydroxide treating solution with acomposition as indicated by point G in Figure I, and 10 standard cu. m.of air are introduced continuously per hour via lines 2, 3, and 4,respectively, into a propeller mixer 1, While stirring vigorously.During the residence time in the propeller mixer, i.e., 6 minutes, themercaptans are almost completely oxidized to disulfides. The mix ture ofgasoline and hydroxide solution is drawn oil continuously via line intosettling tank 6, from which the gasoline with a \mercaptan sulfurcontent of less than 0.0005 by weight is led via line 7 to storage orblending operations (not shown).

The used cresolate-containing potassium hydroxide treating solution isdrawn oil from the bottom of the settling tank 6 via line 8 to aregenerator tower 9. This tower contains 8 grids on each of which is alayer of the regenerating agent, which in this case is solid potassiumcresolate in crystal form free from water of crystallization with acomposition which in Figure I is indicated by point F. The solutionleaving the settling tank 6 flows down over these grids, giving up waterto the solid cresolate. At the bottom of the tower 9 there is obtained asolution which has a composition which is indicated by point G in FigureI. The thus regenerated treating solution is again led to the propellermixer via the line 3' for use in treating further amounts ofmercaptan-containing gasoline. When the mass free from water ofcrystallization on the grids in tower 9 has been completely orsubstantially converted into a mass containing water of crystallization,the water content of the solution returning to the mixer via line 9 willrise because of the decreased absorption of water from the treatingsolution by the exhausted regenerating agent. At this point line 8 isclosed and subsequent amounts of the used treating solution are led vialine 10 to tower 11. This tower has the same construction and capacityas tower 9. If desired, tower 13 can also be connected via line 12. Thistower is also similar to tower 9. If desired, more than one tower may beused at the same time for regenerating the solution used according tothe invention. Exhausted treating agent in the tower is replaced by afresh mass or reactivated in the tower itself, for example, by heatingto drive off the water of crystallization absorbed from the usedtreating solution. It is then suitable for further use in regeneratingused treating solution as before.

We claim as our invention:

1. In a process for removing mercaptans from a light 1 hydrocarbon oilthrough intimate liquid contacting of the oil in presence of oxygen withan aqueous treating solution of an alkali metal hydroxide and an alkalimetal phenolate to effect an oxidation of the mercaptans to disulfideswith an accompanying formation of water of reaction which causes anobjectionable dilution of the treating solution, the improvementcomprising utilizing in the process a treating solution initiallysaturated in the alkali metal phenolate, recovering the water-dilutedtreating solution from the light hydrocarbon oil, passing said recoveredtreating solution into contact with a solid mass of the alkali metalphenolate with the mass initially containing less than the amountofwater of crystallization that it is capable of retaining, holding thetreating solution in contact with said mass until a heterogeneousequilibrium is reached therebetween and there has geen a transfer of theexcess water from the treating solution to the solid mass where it isretained as water of crystallization, said transfer of said excess waterfrom the diluted treating solution being achieved without resort todistillation, separating the regenerated treating solution saturated inthe alkali metal phenolate from the solid mass, and recycling saidtreating solution into contact with additional amounts of the lighthydrocarbon oil.

2. A process in accordance with claim 1, wherein the.

alkali metal is potassium.

3. A process in accordance with claim 2, wherein the light hydrocarbonoil has -a boiling range within the boiling range of gasoline andkerosene, and, before it is contacted with said aqueous treatingsolution, has a mercaptan sulfur content not greater than 0.05% byweight.

4. A process in accordance with claim 3, wherein said light hydrocarbonoil is gasoline and is contacted with said aqueous treating solution ata temperature of from 0 C. to C.

5. A process in accordance with claim 2, wherein the phenolate is acresolate.

'6. A process in accordance with claim 5, wherein the solid massconsists of solid potassium cresolate which initially containssubstantially no water of crystallization.

References Cited in the file of this patent UNITED STATES PATENTS2,556,438 Parker et al June 12, 1951 2,645,602 Tom et al July 14, 19532,663,674 Krause et a1. Dec. 22, 1953

1. IN A PROCESS FOR REMOVING MERCAPTANS FROM A LIGHT HYDROCARBON OILTHROUGH INTIMATE LIQUID CONTACTING OF THE OIL IN PRESENCE OF OXYGEN WITHAN AQUEOUS TREATING SOLUTION OF AN ALKALI METAL HYDROXIDE AND AN ALKALIMETAL PHENOLATE TO EFFECT AN OXIDATION OF THE MERCAPTANS TO DISULFIDESWITH AN ACCOMPANYING FORMATION OF WATER OF REACTION WHICH CAUSES ANOBJECTIONABLE DILUTION OF THE TREATING SOLUTION, THE IMPROVMENTCOMPRISING UTILIZING IN THE PROCESS A TREATING SOLUTION INITALLYSATURATED IN THE